By your description, it sounds like you hear when the car is not rolling. If you only hear it the car is moving or only when you're braking, it is likely something else.
If the squealing is constant with the engine running, it could be a bearing in one of the accessories failing (alternator, water pump, steering pump, pulley, etc). You'll want to use a stethoscope to isolate the noise. Placing your ear on a long screwdriver works but be very careful of moving parts and the cooling fans. Cooling fan can turn on without notice! A long piece of hose actually works really well for this as well to probe for the noise source